This relates generally to electronic devices, and, more particularly, to electronic devices with displays.
Electronic devices such as cellular telephones, computers, and other electronic equipment often contain displays. A display such as a touch screen display includes an array of pixels for displaying images to a user and touch sensor electrodes such as capacitive touch sensor electrodes for gathering touch input from the user. Touch sensor circuitry is used to process touch sensor signals from the touch sensor electrodes. Display driver circuitry such as source line driver circuitry may supply data signals to the array of pixels. Gate line driver circuitry can be used to assert a gate line signal on each row of pixels in the display in sequence to load data into the pixels.
In displays such as touch screen displays, there is a potential for the pixel array to create interference for the touch sensor. The signal-to-noise ratio of the touch sensor can be enhanced by periodically pausing the operation of the display. Pause operations that have the potential to take place in the midst of displaying a frame of data on the display are sometimes referred to as intraframe pause operations.
During an intraframe pause, gate line signals, data signals, and clock signals are held at direct current (DC) levels. This reduces noise and enhances the signal-to-noise ratio for the touch sensor, but has the potential to create undesired voltage stress on certain transistors in the gate line driver circuitry. The gate line driver circuitry includes shift register circuitry that is based on a series of coupled gate line driver stages. The output of each stage is typically used both to drive the gate line associated with that stage and to serve as a carry signal for a subsequent stage. If care is not taken, an output transistor in a paused gate line driver stage will be subjected to a long voltage stress during pausing, because the output of the previous stage will be supplied to the gate of the output transistor until the intraframe pause is over. This stress is different from the voltage stress experienced by the output transistors in the gate line driver stages that have been turned off. Unequal transistor stresses such as these can result in undesired visible artifacts such as dim lines on a display.
It would therefore be desirable to be able to provide a display such as a touch screen display having gate line driver circuitry that minimizes or avoids unequal transistor stresses.